[CWG2091] template parameter lookup differs from g++ wrt. constness

[dommldomml]

Bugzilla Link	42888
Version	8.0
OS	Linux
Attachments	Source code illustrating the problem
CC	@DougGregor,@zygoloid

Extended Description

For the attached source file, clang++ errors out with

main.C:18:4: error: invalid operands to binary expression ('AA' and 'S<E, EV>')
        a << x;
        ~ ^  ~
main.C:10:14: note: candidate template ignored: substitution failure [with E = E]: deduced non-type template argument does not have the same type as the corresponding template parameter ('E *' vs 'const E *')
                friend AA& operator<<( AA& os, const S<E,V>& e );

g++ handles the template fine.

Removing the constness of the second operator parameter or adding constness to the variable makes the problem disappear.

In my application a different operatow was chosen leading to a misbehavior.

[zygoloid]

I think this is due to a recent language change (bugfix), http://wg21.link/cwg2091, that GCC implements and Clang does not yet.

[dommldomml]

This code exists in our code base since 2017. We use gcc 7.x now, back then is was most probably gcc 6.x

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: None (a5fec13e-39c8-49b2-b3dd-e7d891b4dfc7)

| | | | --- | --- | | Bugzilla Link | [42888](https://llvm.org/bz42888) | | Version | 8.0 | | OS | Linux | | Attachments | [Source code illustrating the problem](https://user-images.githubusercontent.com/92601461/143759583-6646354e-dd8b-49bf-9da8-8fbfe04b177d.gz) | | CC | @DougGregor,@zygoloid |

Extended Description

For the attached source file, clang++ errors out with

main.C:18:4: error: invalid operands to binary expression ('AA' and 'S<E, EV>')
        a << x;
        ~ ^  ~
main.C:10:14: note: candidate template ignored: substitution failure [with E = E]: deduced non-type template argument does not have the same type as the corresponding template parameter ('E *' vs 'const E *')
                friend AA& operator<<( AA& os, const S<E,V>& e );

g++ handles the template fine.

Removing the constness of the second operator parameter or adding constness to the variable makes the problem disappear.

In my application a different operatow was chosen leading to a misbehavior.

[cor3ntin]

Fixed in clang 10
https://godbolt.org/z/G35dv4boz

@Endilll Can you add a test for CWG2091? Thanks